1. Field of the Invention
The present invention relates to an oscillator using a MEMS technology.
2. Description of the Related Art
Under the increasing demand for reduction in size and increase in accuracy of electronic devices such as personal computers and wireless mobile devices represented by cellular phones, a small and stable high frequency signal source is inevitable in these electronic devices. A crystal resonator is a representative electronic part that satisfies the demand. It is known that the crystal resonator has an extremely high resonance sharpness (that is, Q value) which is an index of the oscillation device quality, and which exceeds 10,000 owing to the excellent crystal stability. This is a reason that the crystal resonator is widely used as a stable high frequency signal source of the wireless mobile devices, the personal computers, or the like. However, it has been also proved that the crystal resonator cannot sufficiently satisfy a recent demand for further downsizing of the electronic devices.
Under the above circumstances, in recent years, there has been reported a MEMS oscillator using, instead of a crystal resonator, a downsized MEMS resonator that is formed with technology for micro electro mechanical system (MEMS) using a silicon substrate (refer to US 2006/0033594 A1). A MEMS oscillator can be made smaller in size than the crystal oscillator, and can also operate in high frequency. Accordingly, MEMS resonators are expected to spread particularly into compact devices such as a cellular phone. Also, it is possible to integrate a MEMS resonator and a peripheral circuit into a single chip since the MEMS resonator can be manufactured from a silicon substrate.
FIG. 9 is a principle diagram showing a MEMS resonator. As shown in the figure, the MEMS resonator is disposed opposite to a substrate 10 at a distance of a gap 14 in a floating state to form an oscillation beam 11. Both ends of the oscillation beam 11 are fixed to the substrate 10 through anchors 16. A drive electrode 12 and a sense electrode 13 are disposed opposite to each other with capacitive gaps 15 with respect to the oscillation beam 11, respectively. Since the MEMS resonator is driven with an electrostatic force generated by applied voltage, in which application of a DC bias voltage in addition to an AC signal can draw the same electric characteristics (for example, a Q value) as that of a crystal resonator, it is possible to use a MEMS resonator as a resonator of a oscillator with the same configuration as that of the crystal oscillator. The oscillation frequency f0 of the MEMS resonator can be represented by using an effective mass Meff and an effective hardness Keff as follows.f0=1/(2π)√(Keff/Meff)
Since material of the oscillation beam 11 expands and contracts according to the environmental temperature, the effective mass Meff changes. And since the Young's modulus of the material for the oscillation beam 11 changes according to the environmental temperature, the effective hardness Keff also changes.
In the MEMS oscillator according to the above conventional art, however, there arises such a problem that the temperature dependency of the oscillation frequency is worse than that of a crystal resonator. This is because the effective mass and the effective hardness change according to the temperature. To correct the temperature dependency a temperature sensor is required though, there arise such problems that the size of the oscillator becomes large, and the cost also increases when a temperature sensor is incorporated separately into the oscillator. Disposition of the temperature sensor at a position apart from the resonator arises another problem that a error in the temperature detection becomes so large that deteriorates accuracy of correction.